The present invention relates to telecommunications in general, and, more particularly, to a technique for using cascaded polyphase DFT-filter bank in software-defined radios to support wireless telecommunications.
FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-4) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center (xe2x80x9cWSCxe2x80x9d) 120, which may also be known as a Mobile Switching Center (xe2x80x9cMSCxe2x80x9d) or Mobile Telephone Switching Office (xe2x80x9cMTSOxe2x80x9d). Typically, Wireless Switching Center 120 is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic area serviced by the system and to the local and long-distance telephone and data networks (e.g., local-office 130, local-office 138 and toll-office 140). Wireless Switching Center 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal (e.g., wireline terminal 150), which is connected to the system via the local and/or long-distance networks.
The geographic region serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called xe2x80x9ccells.xe2x80x9d As depicted in FIG. 1, each cell is schematically represented by a hexagon; in practice, however, each cell usually has an irregular shape that depends on the topography of the terrain serviced by the system. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center 120.
For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the desired information to base station 103-1, which relays the information to Wireless Switching Center 120 over wireline 102-1. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal 101-2, Wireless Switching Center 120 then returns the information back to base station 103-1 over wireline 102-1, which relays the information, via radio, to wireless terminal 101-2.
A base station will typically receive numerous communications from a number of wireless terminals that are located in the cell serviced by the base station. These numerous communications are received as an analog wide-band radio frequency (RF) signal at the base station. As used herein, the term xe2x80x9cwide-bandxe2x80x9d refers to a band or range of radio spectrum that contains multiple narrow-bands. As used herein, the term xe2x80x9cnarrow-bandxe2x80x9d refers to a carrier band, which has a specified bandwidth for modulation and demodulation. Such carrier bands or specified bandwidths are specific to different communications standards. For example, a narrow-band is defined as 30 kHz for TDMA (IS-136), and a signal of 15 MHz would be a wide-band signal because it would have 500 narrow-bands for the TDMA system (500=15 MHz/30 kHz).
The analog wide-band RF signal is then typically separated by frequency into narrow-band channels at the base station. Individual communications contained in the narrow-band channels are then further processed within the telecommunications system.
One technique in the prior art for processing the analog wide-band RF signal is through the use of a software-defined receiver at the base station. In this prior art technique, the software-defined receiver will often contain, among other things, an analog-to-digital converter for converting an analog signal into a digital signal and a polyphase filter bank for separating the digital signal into narrow-band channels. Each narrow-band channel comprises a xe2x80x9cpass-bandxe2x80x9d (representing a frequency band containing information associated with the narrow-band channel), a xe2x80x9cstop-bandxe2x80x9d (representing a frequency band that does not contain such information) and a xe2x80x9ctransition-bandxe2x80x9d (representing a frequency band between the pass-band and the stop-band). The purpose of the polyphase filter bank is to organize information contained in the digital signal into appropriate xe2x80x9cpass-bandsxe2x80x9d of the narrow-band channels.
A schematic diagram of a polyphase filter bank is shown in FIG. 2. The digital signal is divided into a number, M, of branches by decimating the digital signal on a time basis. Decimating a digital signal decreases the sampling rate of such signal typically through a process of filtering and downsampling. If a digital signal has a sampling rate of R, a decimator will decrease the sampling rate by a factor, D, to produce a new sampling rate of R/D. For example, when a signal has a sampling rate of 9 and is decimated by a factor of three, the decimator will form a new signal with a sampling rate of 3. In this example, a decimator performs integer decimation because the D factor is an integer. Fractional decimation is also possible and is typically achieved through a combination of decimation and interpolation, which will be described below.
Each branch contains a Finite Impulse Response Filter (FIR) through which the decimated digital signals are filtered. The decimated digital signals are stored in locations or xe2x80x9ctapsxe2x80x9d within the FIR filters. The Crochiere and Rabiner equation provides the number, N, of FIR taps required for filtering such decimated digital signals.
[1]      N    ≅                            D          ∞                ⁢                  (                                    δ              p                        ,                          δ              s                                )                            Δ        ⁢                  xe2x80x83                ⁢                  F          /          F                      ,
where:
xcex4p is the xe2x80x9cripplexe2x80x9d or mean amplitude of the signal in the pass-band,
xcex4s is the xe2x80x9cripplexe2x80x9d or mean amplitude of the signal in the stop-band,
D∞(xcex4p,xcex4s)=log10xcex4s*[0.005309*(log10xcex4p)2+0.07114*log10xcex4pxe2x88x920.4761]xe2x88x92[0.00266*(log10xcex4p)2+0.5941*log10 xcex4p+0.4278],
xe2x80x9c*xe2x80x9d indicates multiplication,
xcex94F is the bandwidth of the transition-band in Hz, and
F is the sampling rate of a FIR filter in Hz.
The output digital signals from the FIR filters enter a Discrete Fourier Transform (DFT), such as a Fast Fourier Transform (FFT), where the separate digital signals are organized into M channels. Such an arrangement of FIR filters followed by a FFT transform is called a polyphase filter bank.
As illustrated in FIG. 3, the polyphase filter bank can be cascaded where polyphase filter banks are repeated for several stages, forming a cascaded polyphase DFT-filter bank, to transform a wide-band digital signal into a large number of narrow-band channels. A large number of narrow-band channels are not typically formed within a single polyphase filter because the size of that polyphase filter bank would become too large to effectively process the numerous communications.
Similarly, a polyphase filter bank or a cascaded polyphase DFT-filter bank can be used in a software-defined transmitter. As shown in FIG. 4, M narrow-band channels are combined into a single digital signal through use of an inverse Fast Fourier Transform (IFFT) or an inverse Discrete Fourier Transform (IDFT) methods and interpolating the digital signal on a time basis, in well-known fashion. Interpolating a digital signal increases the sampling rate of such signal typically through a process of upsampling and filtering. If a digital signal has a sampling rate of R, an interpolator will increase the sampling rate by a factor, L, to produce a new sampling rate of R*L. For example, when a signal has a sampling rate of 9 and is interpolated by a factor of three, the interpolator will form a new signal with a sampling rate of 27. In this example, the interpolator performs integer interpolation because the L factor is an integer. Fractional interpolation is also possible and is typically achieved through a combination of decimation and interpolation.
Operating requirement of a polyphase filter bank or a stage of a cascaded polyphase DFT-filter bank depends mainly upon computational rates performed within a polyphase filter bank and to a lesser degree upon storage requirements of a polyphase filter bank. As used herein, xe2x80x9coperating requirementxe2x80x9d refers to the computational rates of a polyphase filter bank or a cascaded polyphase DFT-filter bank. Such computational rates and storage requirements can be defined and minimized for a single polyphase filter or for each stage of a cascaded polyphase filter by well known Crochiere and Rabiner techniques. The computational rate, RT, to be minimized is defined by
[2]            R      T        =                            D          ∞                ⁢                  (                                                    δ                p                            /              K                        ,                          δ              s                                )                    ⁢                        ∑                      i            =            1                    K                ⁢                  xe2x80x83                ⁢                                            F                              i                -                1                                      *                          F              i                                                          F              i                        -                          F              s                        -                          F              p                                            ,
where
K is the number of stages in the cascaded polyphase filter,
s is the stop-band,
p is the pass-band,
xcex4p is the xe2x80x9cripplexe2x80x9d or mean amplitude of the signal in the pass-band,
xcex4s is the xe2x80x9cripplexe2x80x9d or mean amplitude of the signal in the stop-band,
i is a stage of the cascaded polyphase filter (ixe2x89xa6K),
F is frequency in Hz, and
D∞(xcex4p/K, xcex4s) is defined as in equation 1, except that xcex4p is replaced with xcex4p/K.
The storage requirement, NT, to be minimized is defined by
[3]            N      T        =                            D          ∞                ⁢                  (                                                    δ                p                            /              K                        ,                          δ              s                                )                    ⁢                        ∑                      i            =            1                    K                ⁢                  xe2x80x83                ⁢                              F                          i              -              1                                                          F              i                        -                          F              s                        -                          F              p                                            ,
where the terms are defined as in equation 2.
This prior art technique, however, does not investigate the cascading of a plurality of polyphase filters, nor does it examine the overall-operating requirement of a cascaded polyphase DFT-filter bank in terms of radio spectrum, which is used to receive or transmit communications. In other words, in the prior art, a stage of a cascaded polyphase DFT-filter bank has been optimized, but the input or output signals of a base station (e.g., the radio spectrum) have not been examined to reduce the operating requirement of a cascaded polyphase DFT-filter bank. Reducing the operating requirement of individual stages of a cascaded polyphase DFT-filter bank does not necessary result in an overall efficient cascaded polyphase DFT-filter bank because the nature of the cascading and the amount of radio spectrum enter into the overall operating requirements of a cascaded polyphase DFT-filter bank.
The art would benefit from a technique for reducing the operating requirement of a cascaded polyphase DFT-filter bank in a software-defined radio that considers overall computational requirements, cascading schemes and radio spectrum. Such a software-defined radio reduces costs associated with receiving and transmitting signals in a wireless telecommunications system.
In some embodiments, the present invention provides a telecommunications system that uses a cascaded polyphase DFT-filter bank having reduced computational requirements, as compared to the prior art, for receiving and transmitting communications. The computational requirements of the cascaded polyphase DFT-filter bank are reduced by specifically selecting a radio spectrum in which the number of channels in the radio spectrum can be factorized into small prime numbers.
In one embodiment of the present invention, the number of channels received at a base station are specifically selected with a goal of reducing computational requirements of the cascaded polyphase DFT-filter bank. The cascaded polyphase DFT-filter bank is then designed for this selected number of channels.
An illustrative method in accordance with the present teachings comprises the operations of:
selecting a first number, MA, of narrow-band channels based on a nominal amount of spectrum (e.g. a 15 MHz analog wide-band signal) and a narrow-band bandwidth as dictated by communications system standards (e.g. ,30 KHz for TDMA (IS-136));
selecting a second number, MB, of narrow-band channels, where MBxe2x89xa7MA, wherein the second number, MB, of narrow-band channels results in a minimum operating requirement for the cascaded polyphase DFT-filter bank over a range of narrow-band channels evaluated;
defining a second analog wide-band signal based on the second number, MB, of narrow-band channels and the narrow-band bandwidth;
receiving the second analog wide-band signal at a base station; and
converting the second analog wide-band signal into MB narrow-band channels.